Electricity Cost Calculator

Last updated: 2026-05-12

The Four Numbers You Need

Every electricity cost calculation comes down to four inputs:

1. Power draw (watts) — how much electricity the appliance uses while running, found on the product label or spec sheet.
2. Daily hours of use — how long you actually run it each day.
3. Days used per year — how many days a year it actually runs. Use 365 for an always-on device; use fewer for a seasonal one.
4. Electricity rate — what your utility charges per kilowatt-hour (kWh), shown on your bill.

Once you have those four numbers, the math is straightforward.

The Formula

Where $P$ is the power draw in watts, $h$ is the daily use in hours, $d$ is the days used per year, and $p$ is the unit price per kWh.

The divide-by-1,000 step converts watts to kilowatts, because utility bills are billed in kilowatt-hours, not watt-hours. One kilowatt-hour is the energy a 1,000 W appliance uses in exactly one hour.

Why 30 Days for the Month?

Utility bills and appliance energy labels use a 30-day month as a universal approximation. Real months have 28 to 31 days, but a fixed 30-day baseline makes comparisons between different appliances and different countries consistent. The monthly figure assumes the appliance runs every day.

Why a Separate Days-Per-Year Input?

A fixed 30-day month works for an always-on device, but it overstates the cost of a seasonal appliance. An air conditioner used 100 days a year does not cost "monthly cost × 12" — it costs far less. The days used per year input fixes this: the yearly figures multiply the daily cost by exactly the number of days the appliance runs. For a fridge, leave it at 365; for a summer-only air conditioner, drop it to 90–120.

Worked Example: Air Conditioner

A mid-size split air conditioner draws 800 W while running. During summer you run it 6 hours per day, across roughly 120 days of the cooling season. Your electricity rate is ¥30/kWh.

Running the air conditioner costs roughly ¥4,300 for each month of active use, and about ¥17,300 across a full cooling season. Note the difference from a naive "monthly cost × 12" of ¥51,840 — that would assume the unit ran every day of the year. The days-per-year input is what keeps the annual estimate honest. A single air conditioner can still account for 30–50% of a typical household electricity bill during peak summer.

Reading the Wattage Label

The power figure you need is on the compliance label, usually near the power cord or on the bottom of the appliance. Look for a number followed by W or kW. A few things to watch for:

• Range vs. single value. Some appliances list a range (e.g. "60–1,200 W"). For a microwave, 1,200 W is the cooking output at full power; the actual draw over a typical use cycle is lower. Use the higher figure for a conservative estimate.
• Input vs. output. Solar panels, motors, and amplifiers sometimes show both an input (electrical consumption) and an output (mechanical or acoustic power). Use the input power for electricity cost.
• kW vs. W. Multiply kW by 1,000 to get W before entering it, or just switch the unit in the calculator.

Typical Wattages for Common Appliances

Figures are indicative and vary by model and usage mode. Check your own appliance's label for an accurate number.

Variable-Wattage Appliances

Many appliances do not run at a fixed wattage:

• Air conditioners and heat pumps cycle the compressor; effective power is often 60–80% of the rated maximum.
• Refrigerators run the compressor intermittently; average draw is typically 30–50% of the rated peak.
• Inverter washing machines draw more during the spin cycle and less during soaking.

For these devices, either use the "typical" or "average" wattage from the spec sheet (many modern appliances include this separately from the peak rating), or use the nameplate figure as an upper bound and understand your estimate will be conservative.

Electricity Rates Around the World

Electricity rates vary enormously by country and even by region within a country.

Find your exact rate on your utility bill or your energy provider's website. If you are on a time-of-use tariff, use the rate that applies when the appliance mostly runs.

Practical Uses

Prioritizing which appliances to upgrade

Running this calculation for every major appliance in your home quickly reveals where the money goes. High-wattage devices running many hours per day dominate the bill. A refrigerator running 24 hours is often the third or fourth biggest load behind HVAC, even though its peak wattage looks modest.

Comparing old vs. new models

The EU, US (Energy Star), Japan (Top Runner), and many other programs mandate energy labels showing annual consumption in kWh. The yearly consumption output of this calculator is the same quantity, so you can compare a label figure against your own usage pattern directly — set days used per year to 365 if the label assumes year-round use. Multiply by your rate to get annual cost in your local currency, and the payback period on a more efficient appliance is then straightforward arithmetic.

Understanding standby power

Many appliances draw power even when "off" — televisions, game consoles, chargers, and set-top boxes are common culprits. Standby draws are typically 1–10 W. Small per device, but spread across 20+ devices in a home and running 8,760 hours a year, they add up. To include standby in your estimate, run a separate calculation for the standby wattage × 24 hours and add it to the active use cost.